There is known from U.S. Pat. No. 4,688,036, a “keyless” access system for a motor vehicle, wherein the electronic lock control device, in particular the electromagnetic signal emission and reception circuits, is activated only after the user has activated a manual switch. The user in possession of an authorised transponder, thus actuates a switch arranged outside the car to activate first of all the transmitter of the control device such that the latter can send an interrogation signal to the transponder after having woken it. Thus, activation of the system's emission and reception circuits and the communication protocol for determining whether the transponder is authorised occur between actuation of the external switch and the sending of an unlocking signal to the electronic lock.
By providing an external manual switch in the method for opening a door using a transponder, the power consumption of the door locking control device is limited, this control device being activated only for a given period of time following actuation of the external switch.
It will be noted in fact, that continuous activation of the control device, particularly periodically sending an interrogation signal to detect the presence of an authorised transponder, generates high energy consumption, particularly because of certain materials that form the car. Although the energy source in a car can be large, particularly taking energy from a battery of the latter, continuous activation of the electromagnetic signal transmission means can cause the battery in question to run down when the car is not used regularly.
U.S. Pat. No. 4,688,036 provides for the use of only one communication frequency and for the external switch to be arranged under the handle of the door concerned.
DE Patent No. 198 27 586 discloses a more improved access system to a car, wherein low frequency (LF) and high frequency (HF) communication is provided, so as to increase the transmission speed of encoded data between the electronic door unlocking device and an authorised transponder, the validly detected presence of which causes said door to open. The speed of such communication is particularly important within the scope of the main proposed embodiment where the transmission circuit for an interrogation signal is activated by actuating the handle used for opening the door. It is thus preferable, in such an embodiment, for the electronic lock to be opened quickly; otherwise the user will find the door locked first of all when he pulls the handle to open the door.
It will be noted that the low frequency transmitter is essentially used for sending the interrogation signals to the transponders in a limited area, close to the door.
As shown in FIG. 1 annexed hereto, the reception area for an LF interrogation signal by a transponder defines approximately a semi-circle 2 with a radius of 1.5 to 2 m around antenna 6 arranged in a door of car 8. However, the high frequency communication circle 4 between transponder 10 and the HF receiver arranged in the car typically has a radius of at least 30 m.
DE Patent No. 198 27 586 also provides for a remote actuation button for the electronic lock to be arranged in the transponder. Thus, when the user wishes to open a door or the doors of his car from a distance, he only need depress the button-switch of the transponder for the latter to send an HF signal to the HF reception means of the lock control device. This improved embodiment requires the HF reception means of the control device to be permanently or almost permanently in listening mode. Although this function requires a certain amount of energy, the energy consumption generated is however relatively low for the conventional energy sources that can be arranged in cars.
DE Patent No. 198 27 586 also proposes a particular embodiment wherein no external switch is provided for activating the lock control device communication means, the low frequency transmission means continuously sending, in a cyclical manner, an interrogation signal. Given that the communication radius is typically from 1.5 to 2 m for the LF transmitter, the door opening authorisation protocol can be started as soon as a transponder enters the LF communication area. When a user provided with a transponder actuates an external switch of the vehicle, the transponder is located between 0.5 and 1 m from the LF transmitter. In the case of this particular embodiment, starting from the hypothesis that the user walks at a speed of 1 m per second, the door opening authorisation protocol can start approximately one second before the moment when the user reaches the door handle. Thus, the communication time available is greatly increased such that it is possible to implement a highly secure identification protocol and to unlock the electronic lock before the user pulls the handle to open the door. This feature increases the ease of use of the “keyless” opening system. However, currently, this embodiment is not used in practice since it generates high energy consumption, as was explained hereinbefore. Given this major problem, automobile manufacturers generally provide for the doors of a car to be opened either by actuating the handle, or by pressing on a button-switch arranged on the surface of the transponder.
It is an object of the present invention to overcome the aforementioned problem while maintaining optimum use of the low frequency communication area of the control device.